Practice question

RNA polymerase I is localized to fibrillar center of nucleolus and devoted exclusively to ribosomal RNA production, accounting for up to sixty percent of total cellular transcription in proliferating cells. It transcribes a single tandem repeat locus into large 47S precursor in mammals, 45S in mice, containing 18S, 5.8S, and 28S sequences separated by external and internal transcribed spacers. Spacer regions are removed by endo and exonucleases guided by snoRNAs. Promoter includes core element and upstream control element bound by selectivity factor SL1 and UBF architectural factor.